I have a friend that has a holiday home in northern Catalonia which is off grid but he would like to have internet access, does any of our Spanish based contributors have any idea of the best mac friendly dongles and which carrier might be the best?

No idea about carriers, but I have come to the opinion that devices like the Mifi are a much better idea than USB dongles. The software for the latter is often cludgy in the extreme, while a Mifi-like device allows you to use your laptop's ordinary wireless network access, indeed usually up to five can be connected.
Sim-free devices are available, so you can use them with any carriers sim card in which ever country you happen to find yourself.
I shall be buying one when I can afford to do so.

Thanks for that Rhakios, I had completely forgotten about them; one of the guys from my LUG demonstrated his device from 3 a couple of months ago, there were two other people with smart phones that he gave access to the mifi successfully

I have no idea what mifi means, but my house has no landline so I use a radio system which I access via eth0 router and it suits me fine. There is also the Motorola Canopy system available in some areas, which gives more bandwidth and no contention issues. Same eth0 router access.

I live on the Costa Blanca, about 400km South of Catalonia.

Haakin lives in Lleida, which is Catalonia, so he may be better placed to advise, if he reads this.